Adaptive filter networks have found application in a wide variety of arrangements. For example, J. L. Kelly, Jr. and B. F. Logan in U.S. Pat. No. 3,500,000 issued Mar. 10, 1970 disclose an echo canceller employing estimator apparatus including an adaptive filter for synthesizing a linear approximation of the echo. Using a tapped delay line having an integer N taps spaced along its length at convenient Nyquist intervals, the filter develops a number of delayed replicas of the incoming signal. The delayed replicas are correlated, e.g., by multiplication with an error signal, and the resultant correlated signal is averaged, e.g., using an intergrator, to provide a tap coefficient signal whose polarity and magnitude indicate the appropriate adjustment for gain controlling networks. The gain controlled output signals are then algebraically combined to obtain the synthesized echo estimate for subtraction from the outgoing signal. Thereby, the echo is cancelled.
Another application for adaptive filter networks is in the automatic equalizer art. An automatic equalizer generally includes an adaptive transversal filter in which successive, i.e., delayed, samples of the incoming signal are multiplied by respective tap coefficients and the products summed to obtain an equalized signal. An error signal equal to the difference between the equalized signal and a reference signal is formed. The error signal is used to adjust the equalizer by updating the tap coefficients in such a way as to minimize signal distortion.
Unfortunately, biases in the tap coefficient providing apparatus, for example, updating biases associated with rounding signal values consistent with the precision of the apparatus being employed, can cause some tap coefficients to drift toward relatively large values in which event the coefficients are said to "blow up". To mitigate blow up, tap coefficient leakage of the type disclosed in a copending application of R. D. Gitlin et al Case 5-5-7, entitled "Coefficient Tap Leakage for Fractionally-Spaced Equalizers", Ser. No. 16,495, and filed Mar. 1, 1979 is introduced for driving some tap coefficients toward zero. Clearly, if all coefficients are driven to zero, the functional operativeness of the echo canceller or the automatic equalizer would be circumvented. Hence, tap coefficient leakage must not be too "strong".